The liquefaction of coal to yield liquid and gaseous hydrocarbons has been known for some time. The main objective of coal liquefaction is to convert coal into a more efficient fuel that burns cleaner and is easier and less costly to transport. During the liquefaction process, the macromolecular network of the coal is broken into smaller units resulting in lighter products of reduced molecular weight. This involves an upgrading in the hydrogen content of the resulting products. Basically, liquefaction is accomplished by rapidly heating coal, slurried in a hydrogen donor vehicle, for considerably long residence times.
One of the earliest reported coal liquefaction processes was the Bergius process (1914) which used a paste of coal, heavy oil, and a small amount of iron oxide catalyst at 450.degree. C. and 200 atmospheres in a stirred autoclave. This process was later refined by I. G. Farben in Germany to produce commercial quality gasoline during World War II. A similar process was used in Great Britain, developed by Imperial Chemical Industries, to hydrogenate coal to make gasoline but this process has not been in commercial use since 1958 and no process of coal hydrogenation is used commercially in either England or Germany today.
In the United States the solvent refining of coal was developed during the energy crisis in the late 1950s and a pilot demonstration plant was constructed which had an output of 45 tons per day, utilizing a slurry of coal which was taken up in process-derived anthracene oil and heated to about 425.degree. C. at about 2000 psi of hydrogen for about 1 hour. After filtration, the major product produced was a low ash, low sulfur, tar-like heavy boiler fuel. This pilot plant was never put into commercial production.
Later, the H-Coal process was developed by Hydrocarbon Research, Inc., at Catlettsburg, KY. This pilot plant produced a product from a slurry of crushed coal and recycled oil which was treated with hydrogen in an ebulliated bed reactor at 200 atmospheres and about 455.degree. C. with catalyst. Again, a commercial plant was never built incorporating this technology. More detail on this pilot plant can be found in Kirk-Othmer, Encyclopedia of Chemical Technology, 3d ed., 42-47 (1979).
To date, all existing processes for the direct liquefaction of coal by solvent extraction have utilized molecular hydrogen at high pressures (over 1000 psi). The total hydrogen consumption in these liquefaction processes is in the range of 3-5% of the amount of coal fuel, of which a significant portion comes from molecular hydrogen. The cost analysis of a typical coal liquefaction process shows that as much as one-third of the overall cost is devoted to hydrogen production. The considerable expense of hydrogen production is one of the significant drawbacks to commercial application of coal liquefaction technology.
By the present invention, applicants have developed a process that eliminates the need to use the costly molecular hydrogen in the liquefaction. Applicants have found that methane can be used in place of molecular hydrogen in the liquefaction process without significantly decreasing the yields of the desired oil products. The use of methane, in the form of natural gas, thus permits the use of an abundant natural resource to convert another abundant natural resource, coal, into usable fuel products. Natural gas is less expensive to use than molecular hydrogen derived from the gasification of coal or the reforming of methane.
In contrast to the prior art processes, which all represent hydrogenation processes, the instant invention accomplishes the liquefaction of carbonaceous materials such as coal in a pressurized methane atmosphere. Not only does the instant process have the economical advantages discussed above, it also has an advantage over the conventional hydrogenation approaches in curtailing the amount of unwanted C.sub.1 -C.sub.4 hydrocarbon gas products formed during the liquefaction process. The ability to use the lower temperatures of the present invention in itself decreases the amount of gaseous product formed. In addition, the hydrogenation processes, which involve the hydrogen splitting of the aromatic ring structures in coal, liberate C.sub.1 -C.sub.4 hydrocarbon gases as by-products. The present invention, which proceeds via alkylation reactions rather than hydrogenation, diminishes such splitting reactions and, therefore, decreases significantly the amount of C.sub.1 -C.sub.4 hydrocarbon by-products and the improved yield of liquid products provides an important economic advantage and an improvement in the quality of the higher liquid content end product.
In the practice of the present invention, a preferred carbonaceous material is coal of the bituminous grade. This grade coal is preferred because in its inherent polynuclear structure under high temperature and pressure conditions, there are created the conditions of free radical liberation facilitating a reaction with methane. The methane reactant serves as a reservoir for hydrogen atoms which can be generated in situ to react with the free radicals from the liquefied coal which are developed during the early heating stages of the coal. Although methane gas is homogeneously stable at liquefaction temperatures, thermally produced free radicals from coal and the free radicals from the solvent can abstract a hydrogen atom from methane, thereby setting the stage for a variety of free radical reactions. The role of methane as the hydrogen reservoir becomes more important as the liquefaction process proceeds. During the initial stages of liquefaction, very little hydrogen is required to stabilize the free radicals generated from the coal. However, during the later stages of the liquefaction process, the hydrogen requirement is found to increase exponentially with coal conversion.
The instant invention thus presents a process for liquefying coal that is both efficient and cost effective.